The large molecular weight tracer horseradish peroxidase is injected into groups of rats. The animals are killed after time intervals of from 10 min. to several hours. The enamel organs are fixed, dissected and reacted for peroxidase activity. Tissue blocks are separated as to zones (secretion, maturation, etc.) and sectioned for light and electron microscopy. The movement of tracer through the various zones of the enamel organ is described and correlated with the functions of each zone. BIBLIOGRAPHIC REFERENCES: Kallenbach, E. Fine structure of differentiating ameloblasts in the kitten. Am. J. Anat. 145:283-318 (1976).